1. Field of the Invention
The present invention relates to a molding die for molding a honeycomb structure and also to the honeycomb structure.
2. Description of the Related Art
As is well known, a catalytic converter can be used for purifying exhaust gas emitted from an automobile engine. A honeycomb structure 8 used for catalyst support for such a catalytic converter in automobile comprises, for example, a cell grid 81 to form the honeycomb structure and a skin layer 82 to enclose the outer periphery of the honeycomb structure as shown in FIG. 30. Such a honeycomb structure 8 is produced by an extrusion molding method. Conventional type molding dies used for the above extrusion molding included a die 90, which comprises a metal die 91 for extruding molding material and a guide ring 92 for defining the outer diameter of the honeycomb structure as shown in FIG. 29.
As shown in FIG. 29, the metal die 91 integrally comprises a feed hole forming unit 14 having feed passageways or feed holes 15 for feeding molding material and a slot forming unit 11 where slots 12 communicating with the feed holes 15 are provided. On the other hand, the guide ring 92 has an abutment surface 25 where the material extruded from outer peripheral slot forming unit 130 of outer peripheral portion of the slot forming unit 11 is abutted, and the abutment surface 25 is placed face-to-face to an outer peripheral end surface 13 of the outer peripheral slot forming unit 130 of the metal die 91 with a clearance sector 22 therebetween.
When the honeycomb structure is extruded and molded in this molding die 90, the molding material introduced from the feed holes 15 is extruded in the form of a grid through the slots and a cell grid 81 is formed, and the molding material extruded from the outer peripheral slot forming unit of the slot forming unit 11 is brought into contact with the abutment surface 25 of the guide ring 92. The molding material thus abutted is formed to the skin layer 82 at the clearance sector 22 and is extruded in radially inward direction. Further, it is deflected toward axial direction of the honeycomb structure along an inner guide sector 23 of the guide ring 92, thereby forming the skin layer 82.
Therefore, when the honeycomb structure 8 is formed using this conventional type molding die 90, it is possible to determine the outer diameter of the honeycomb structure 8 depending on the position of the inner guide sector 23 of the guide ring 92. Specifically, by changing the position in radial direction of the inner guide sector 23 of the guide ring 92, the outer diameter of the honeycomb structure can be freely adjusted. Also, the thickness of the skin layer 82 can be determined by clearance distance of the clearance sector 22.
However, there are the following problems in the above conventional type molding die 90 for the honeycomb structure. When the skin layer 82 of the honeycomb structure is designed thick, bending may occur on the inner cell grid 81 when the molding material for forming the skin layer 82 is deflected toward axial direction of the honeycomb structure from the clearance sector 22 along the inner guide sector 23.
A detailed description will now be provided for such a case. In the molding die 90 as described above, the molding material for forming the skin layer 82, which is extruded in radially inward direction of the honeycomb structure 8 is deflected by extrusion force of the molding material, which forms the cell grid 81 extruded through the slots 12. In case the skin layer 82 is thick, component force in radial direction applied on the cell grid from the skin layer 82 becomes too high, and the cell grid 81 is bent inwardly. If bending occurs on the cell grid, compressive breaking strength of the completed honeycomb structure decreases.
Accordingly, the conventional type molding die 90 can be effectively utilized when the skin layer 82 is relatively thin compared with the thickness of the cell grid 81, but it is difficult to suit the molding of the honeycomb structure if the thickness of the skin layer 82 is relatively thick compared with the thickness of the cell grid 81. For example, to meet the requirements of the lightweight type honeycomb structure as produced in recent years, it is necessary to reduce the weight by extensively reducing the thickness of the cell grid and to increase overall strength by increasing the thickness of the skin layer, and it is difficult to use the conventional type molding die 90 for such purpose.
In contrast to this, a molding die for the purpose of preventing the bending of the cell grid is described in each of Japanese Patent Publications Laid-Open 57-157706 and 4-305077. In each of these molding dies, an opposed wall surface placed face-to-face to the inner guide sector 23 is provided, and the skin layer is formed between the inner guide sector 23 and the opposed wall surface.
Although the molding die disclosed in each of the above patent publications is effective for improvement of shape such as bending of the cell grid, outer diameter of the honeycomb structure is determined by the inner guide sector 23 and the opposed wall surface when the thickness of the skin layer is defined by these two components. For this reason, there is a problem in that it is very difficult to change the setting of outer diameter. Specifically, the shrinkage percentage of the product after drying may vary for each material lot, while both metal die and the guide ring must be replaced with those having suitable shrinkage percentage in order to maintain dimensional accuracy of the product.
To solve the above problems, it is an object of the present invention to provide a die for molding a honeycomb structure, which can be molded without causing the bending of the cell grid even when the honeycomb structure has a relatively thick skin layer, and also to provide a honeycomb structure having high strength and without bending on the cell grids.